diff --git a/lib/Kolab/DAV/Auth/HTTPBasic.php b/lib/Kolab/DAV/Auth/HTTPBasic.php
--- a/lib/Kolab/DAV/Auth/HTTPBasic.php
+++ b/lib/Kolab/DAV/Auth/HTTPBasic.php
@@ -175,6 +175,7 @@
         $storage      = $rcube->get_storage();
         $login_lc     = $rcube->config->get('login_lc');
         $default_port = $rcube->config->get('default_port', 143);
+        $username_domain = $rcube->config->get('username_domain');
 
         // parse $host
         $a_host = parse_url($host);
@@ -193,6 +194,24 @@
             $port = $default_port;
         }
 
+        // Check if we need to add/force domain to username
+        if (!empty($username_domain)) {
+            $domain = is_array($username_domain) ? $username_domain[$host] : $username_domain;
+
+            if ($domain = rcube_utils::parse_host((string)$domain, $host)) {
+                $pos = strpos($username, '@');
+
+                // force configured domains
+                if ($pos !== false && $rcube->config->get('username_domain_forced')) {
+                    $username = substr($username, 0, $pos) . '@' . $domain;
+                }
+                // just add domain if not specified
+                else if ($pos === false) {
+                    $username .= '@' . $domain;
+                }
+            }
+        }
+
         // Convert username to lowercase. If storage backend
         // is case-insensitive we need to store always the same username
         if ($login_lc) {